1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to methods and apparatus for providing on-die capacitance.
2. Description of the Related Art
Cascode style and other types of circuits, such as those frequently used in microprocessors, graphics processors and application specific integrated circuits, typically require full-voltage and midpoint-voltage power supply rails. In many conventional semiconductor dice, the on-die decoupling capacitance used to suppress power supply noise is typically formed by stacking capacitors from a ground rail to the mid-voltage rail, from the mid-voltage rail to the full-voltage rail, across the full-voltage rail and ground rail.
The minimum device sizes for integrated circuits have been steadily falling for many years. Shrinking device size has resulted in attendant increases in power density and operating frequencies. Depending on the power density and frequency, current fluctuations during transistor switching can cause voltage bounces on the power rails of a chip. If of sufficient magnitude, the voltage bounces can produce timing errors and even device failure.
One conventional method of addressing voltage irregularities on chip rails involves placing capacitors across the power and ground rails. One conventional variant uses a metal oxide semiconductor (MOS) capacitor design. Many such MOS capacitors may be scattered across various locations of a given die. Ideally, a decoupling capacitor is located in close proximity to a switching site in order to keep disruptive capacitor-to-power rail inductance at acceptable levels. In practice, it is often difficult to provide the desired proximity due to layout constraints in the die.
One conventional technique for improving decoupling involves providing more raw capacitance on the die. However, this technique may be severely constrained by the requirements of packing density or will lead to die size growth.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.